This appears to be due to an increase in system time while using DRM.
On 2.4.21-pre4-ac5:
$ DISPLAY=:0.0 sh -c "vmstat 5 & glxgears"
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 1 0 95544 9952 69252 0 0 368 76 144 288 93 4 4 0
4214 frames in 5.0 seconds = 842.800 FPS
1 0 0 93760 9976 69252 0 0 0 38 104 322 33 67 0 0
5106 frames in 5.0 seconds = 1021.200 FPS
2 0 0 93756 9976 69256 0 0 0 0 102 146 34 66 0 0
5100 frames in 5.0 seconds = 1020.000 FPS
1 0 0 93732 10000 69256 0 0 2 20 103 150 34 66 0 0
5105 frames in 5.0 seconds = 1021.000 FPS
2 0 0 93732 10000 69256 0 0 0 0 101 146 28 72 0 0
5103 frames in 5.0 seconds = 1020.600 FPS
On 2.4.19
$ DISPLAY=:0.0 sh -c "vmstat 5 & glxgears"
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 0 2220 88872 10356 77128 0 2 334 55 197 495 65 32 4 0
2 0 2220 87148 10356 77128 0 0 1 58 118 620 97 3 0 0
4717 frames in 5.0 seconds = 943.400 FPS
1 0 2220 87128 10372 77128 0 0 0 20 103 365 91 9 0 0
1 0 2220 87128 10372 77128 0 0 0 20 102 152 91 9 0 0
6553 frames in 5.0 seconds = 1310.600 FPS
1 0 2220 87112 10388 77128 0 0 0 17 103 147 92 8 0 0
1 0 2220 87112 10388 77128 0 0 0 17 103 148 92 8 0 0
6556 frames in 5.0 seconds = 1311.200 FPS
1 0 2220 87100 10400 77128 0 0 1 2 102 146 92 8 0 0
1 0 2220 87100 10400 77128 0 0 1 2 102 143 92 8 0 0
Hardware is AMD Duron 1.2GHz, VIA KT133A, G400 16MB.
Software is Debian sid, XFree86 4.2.1
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