For fun I ran the TDM 2.06 64 bit Linux release with the INTEL_DEBUG=perf environment variable set. For every rendered frame I saw this output:
intelReadPixels: fallback to CPU mapping in PBO case
CPU mapping a busy miptree BO stalled and took 3.813 ms.
Failed to fast clear 1920x1080 depth because of scissors. Possible 5% performance win if avoided.
I got this on all 3 maps I tested (Training Mission, A New Job, Volta 1). Maybe it's worth investigating (see notes below though).
OS: Debian 9
GPU: Intel HD Graphics 520
Driver version: Mesa 13.0.6
Binary: 64 bit Linux binary
Settings: r_useFBO 1, pretty high settings, soft shadows off
Mesa 13.0.6 is a bit dated by now so I would suggest to verify this with a newer Mesa version if possible.
Note: TDM said that the OpenGL extension EXT_depths_bound_test wasn't found, I'm not sure if this is related to the fast clear problem or not.