diff --git a/ice40/arch.cc b/ice40/arch.cc
index ce45b051..78d4c29c 100644
--- a/ice40/arch.cc
+++ b/ice40/arch.cc
@@ -168,6 +168,9 @@ Arch::Arch(ArchArgs args) : args(args)
     wire_to_net.resize(chip_info->num_wires);
     pip_to_net.resize(chip_info->num_pips);
     switches_locked.resize(chip_info->num_switches);
+
+    // Initialise regularly used IDStrings for performance
+    id_glb_buf_out = id("GLOBAL_BUFFER_OUTPUT");
 }
 
 // -----------------------------------------------------------------------
@@ -501,4 +504,11 @@ bool Arch::isClockPort(const CellInfo *cell, IdString port) const
     return false;
 }
 
+bool Arch::isGlobalNet(const NetInfo *net) const
+{
+    if (net == nullptr)
+        return false;
+    return net->driver.cell != nullptr && net->driver.port == id_glb_buf_out;
+}
+
 NEXTPNR_NAMESPACE_END
diff --git a/ice40/arch.h b/ice40/arch.h
index 07c2582b..a6b70a69 100644
--- a/ice40/arch.h
+++ b/ice40/arch.h
@@ -800,6 +800,10 @@ struct Arch : BaseCtx
     IdString getPortClock(const CellInfo *cell, IdString port) const;
     // Return true if a port is a clock
     bool isClockPort(const CellInfo *cell, IdString port) const;
+    // Return true if a port is a net
+    bool isGlobalNet(const NetInfo *net) const;
+
+    IdString id_glb_buf_out;
 };
 
 NEXTPNR_NAMESPACE_END
diff --git a/ice40/arch_place.cc b/ice40/arch_place.cc
index 60fa928e..7b79e031 100644
--- a/ice40/arch_place.cc
+++ b/ice40/arch_place.cc
@@ -58,11 +58,11 @@ bool PlaceValidityChecker::logicCellsCompatible(
                 clk = get_net_or_empty(cell, id_clk);
                 sr = get_net_or_empty(cell, id_sr);
 
-                if (!is_global_net(ctx, cen) && cen != nullptr)
+                if (!ctx->isGlobalNet(cen) && cen != nullptr)
                     locals_count++;
-                if (!is_global_net(ctx, clk) && clk != nullptr)
+                if (!ctx->isGlobalNet(clk) && clk != nullptr)
                     locals_count++;
-                if (!is_global_net(ctx, sr) && sr != nullptr)
+                if (!ctx->isGlobalNet(sr) && sr != nullptr)
                     locals_count++;
 
                 if (bool_or_default(cell->params, id_neg_clk)) {
@@ -140,7 +140,8 @@ bool PlaceValidityChecker::isValidBelForCell(CellInfo *cell, BelId bel)
     } else if (cell->type == id_sb_gb) {
         bool is_reset = false, is_cen = false;
         assert(cell->ports.at(ctx->id("GLOBAL_BUFFER_OUTPUT")).net != nullptr);
-        for (auto user : cell->ports.at(ctx->id("GLOBAL_BUFFER_OUTPUT")).net->users) {
+        for (auto user :
+             cell->ports.at(ctx->id("GLOBAL_BUFFER_OUTPUT")).net->users) {
             if (is_reset_port(ctx, user))
                 is_reset = true;
             if (is_enable_port(ctx, user))
diff --git a/ice40/cells.cc b/ice40/cells.cc
index 056c33f8..f34b7511 100644
--- a/ice40/cells.cc
+++ b/ice40/cells.cc
@@ -36,8 +36,8 @@ CellInfo *create_ice_cell(Context *ctx, IdString type, std::string name)
     static int auto_idx = 0;
     CellInfo *new_cell = new CellInfo();
     if (name.empty()) {
-        new_cell->name = ctx->id( "$nextpnr_" + type.str(ctx) + "_" +
-                                               std::to_string(auto_idx++));
+        new_cell->name = ctx->id("$nextpnr_" + type.str(ctx) + "_" +
+                                 std::to_string(auto_idx++));
     } else {
         new_cell->name = ctx->id(name);
     }
@@ -269,10 +269,4 @@ bool is_enable_port(const Context *ctx, const PortRef &port)
     return false;
 }
 
-bool is_global_net(const Context *ctx, const NetInfo *net)
-{
-    return bool(
-            net_driven_by(ctx, net, is_gbuf, ctx->id("GLOBAL_BUFFER_OUTPUT")));
-}
-
 NEXTPNR_NAMESPACE_END
diff --git a/ice40/cells.h b/ice40/cells.h
index 3e5f6ebd..29331445 100644
--- a/ice40/cells.h
+++ b/ice40/cells.h
@@ -117,9 +117,6 @@ void dff_to_lc(const Context *ctx, CellInfo *dff, CellInfo *lc,
 // Convert a nextpnr IO buffer to a SB_IO
 void nxio_to_sb(Context *ctx, CellInfo *nxio, CellInfo *sbio);
 
-// Return true if a net is a global net
-bool is_global_net(const Context *ctx, const NetInfo *net);
-
 // Return true if a port is a clock port
 bool is_clock_port(const Context *ctx, const PortRef &port);
 
diff --git a/ice40/pack.cc b/ice40/pack.cc
index 868b0fd8..e6be502f 100644
--- a/ice40/pack.cc
+++ b/ice40/pack.cc
@@ -131,8 +131,9 @@ static void pack_carries(Context *ctx)
         CellInfo *ci = cell.second;
         if (is_carry(ctx, ci)) {
             packed_cells.insert(cell.first);
-            CellInfo *carry_ci_lc = net_only_drives(ctx, ci->ports.at(ctx->id("CI")).net,
-                                                    is_lc, ctx->id("I3"), false);
+            CellInfo *carry_ci_lc =
+                    net_only_drives(ctx, ci->ports.at(ctx->id("CI")).net, is_lc,
+                                    ctx->id("I3"), false);
             if (!ci->ports.at(ctx->id("I0")).net)
                 log_error("SB_CARRY '%s' has disconnected port I0\n",
                           cell.first.c_str(ctx));
@@ -227,7 +228,8 @@ static void pack_ram(Context *ctx)
                               newname.substr(bpos + 1,
                                              (newname.size() - bpos) - 2);
                 }
-                replace_port(ci, ctx->id(pi.name.c_str(ctx)), packed, ctx->id(newname));
+                replace_port(ci, ctx->id(pi.name.c_str(ctx)), packed,
+                             ctx->id(newname));
             }
         }
     }
@@ -268,14 +270,16 @@ static void pack_constants(Context *ctx)
 {
     log_info("Packing constants..\n");
 
-    CellInfo *gnd_cell = create_ice_cell(ctx, ctx->id("ICESTORM_LC"), "$PACKER_GND");
+    CellInfo *gnd_cell =
+            create_ice_cell(ctx, ctx->id("ICESTORM_LC"), "$PACKER_GND");
     gnd_cell->params[ctx->id("LUT_INIT")] = "0";
     NetInfo *gnd_net = new NetInfo;
     gnd_net->name = ctx->id("$PACKER_GND_NET");
     gnd_net->driver.cell = gnd_cell;
     gnd_net->driver.port = ctx->id("O");
 
-    CellInfo *vcc_cell = create_ice_cell(ctx, ctx->id("ICESTORM_LC"), "$PACKER_VCC");
+    CellInfo *vcc_cell =
+            create_ice_cell(ctx, ctx->id("ICESTORM_LC"), "$PACKER_VCC");
     vcc_cell->params[ctx->id("LUT_INIT")] = "1";
     NetInfo *vcc_net = new NetInfo;
     vcc_net->name = ctx->id("$PACKER_VCC_NET");
@@ -336,12 +340,14 @@ static void pack_io(Context *ctx)
             CellInfo *sb = nullptr;
             if (ci->type == ctx->id("$nextpnr_ibuf") ||
                 ci->type == ctx->id("$nextpnr_iobuf")) {
-                sb = net_only_drives(ctx, ci->ports.at(ctx->id("O")).net, is_sb_io,
-                                     ctx->id("PACKAGE_PIN"), true, ci);
+                sb = net_only_drives(ctx, ci->ports.at(ctx->id("O")).net,
+                                     is_sb_io, ctx->id("PACKAGE_PIN"), true,
+                                     ci);
 
             } else if (ci->type == ctx->id("$nextpnr_obuf")) {
-                sb = net_only_drives(ctx, ci->ports.at(ctx->id("I")).net, is_sb_io,
-                                     ctx->id("PACKAGE_PIN"), true, ci);
+                sb = net_only_drives(ctx, ci->ports.at(ctx->id("I")).net,
+                                     is_sb_io, ctx->id("PACKAGE_PIN"), true,
+                                     ci);
             }
             if (sb != nullptr) {
                 // Trivial case, SB_IO used. Just destroy the net and the
@@ -416,7 +422,7 @@ static void promote_globals(Context *ctx)
     std::map<IdString, int> clock_count, reset_count, cen_count;
     for (auto net : sorted(ctx->nets)) {
         NetInfo *ni = net.second;
-        if (ni->driver.cell != nullptr && !is_global_net(ctx, ni)) {
+        if (ni->driver.cell != nullptr && !ctx->isGlobalNet(ni)) {
             clock_count[net.first] = 0;
             reset_count[net.first] = 0;
             cen_count[net.first] = 0;
@@ -503,7 +509,8 @@ static void pack_intosc(Context *ctx)
             replace_port(ci, ctx->id("CLKLFEN"), packed, ctx->id("CLKLFEN"));
             replace_port(ci, ctx->id("CLKLFPU"), packed, ctx->id("CLKLFPU"));
             if (bool_or_default(ci->attrs, ctx->id("ROUTE_THROUGH_FABRIC"))) {
-                replace_port(ci, ctx->id("CLKLF"), packed, ctx->id("CLKLF_FABRIC"));
+                replace_port(ci, ctx->id("CLKLF"), packed,
+                             ctx->id("CLKLF_FABRIC"));
             } else {
                 replace_port(ci, ctx->id("CLKLF"), packed, ctx->id("CLKLF"));
             }