Chris pointed me to the fact, that the window geoms calculation can be done once for each monitor, so I applied this remark
--- a/dwm.c Mon Feb 11 20:51:04 2008 +0000
+++ b/dwm.c Mon Feb 11 20:57:56 2008 +0000
@@ -1724,12 +1724,13 @@
for(n = 0, c = nexttiled(clients, i); c; c = nexttiled(c->next, i))
n++;
+ /* window geoms */
+ mw = (n == 1) ? m->waw : m->mwfact * m->waw;
+ th = (n > 1) ? m->wah / (n - 1) : 0;
+ if(n > 1 && th < bh)
+ th = m->wah;
+
for(j = 0, c = mc = nexttiled(clients, i); c; c = nexttiled(c->next, i)) {
- /* window geoms */
- mw = (n == 1) ? m->waw : m->mwfact * m->waw;
- th = (n > 1) ? m->wah / (n - 1) : 0;
- if(n > 1 && th < bh)
- th = m->wah;
if(j == 0) { /* master */
nx = m->wax;
ny = m->way;