Welcome to the SeeMeCNC Community

This is on a Rostock MAX v2
0.91 firmware
Slic3r RC3 and Stable
PLA
MatterControl and Repetier Host
Direct Connect and SD Card

All objects, but for the sake of testing, used 0.5 mm Thin Wall and Hollow Cube.

No matter what I set the printing speed, the first layer prints at the set speed and the rest of the layers slow down 10 mm/s. The screwy thing is that if I use the slider in either Repetier Host or MatterControl, the printing speeds up.

For testing, I changed all of the Slic3r settings to default and changed all of the speeds to 80 mm/s. The hot ends was set at 210 C. The EEPROM has the maximum speed rate at 300.

The prints are coming out OK, but everything is larger than specified as well as much slower than expected.

Anyone got any advice?

Any ideas?

If things are larger than expected it's a firmware issue, most likely incorrect steps/mm, though incorrect delta arm length can also result in the issue.
As to the running slower, it's almost certainly your slicer config, either minimum layer time set very high, or one of the speed settings, it's possible you may have to delete all your old slicer configs as it may be picking up an older setting if you've upgraded at any point.

If you are running Slic3r from Matter Control, I would say drop it until they implement it better or perhaps use the Cura option. Honestly, I like the people but I can't get great prints out of it - for both the Rostock or the Robo3D...just the calibration cube or those included with the install. Of course, those are vetted STLs...

0.91 is doing a much better job at retaining speeds than previous versions. Polygonhell's suggestion of blasting your configs is a good idea.

Support replied to an e-mail. The speed issue was due to a cooling setting that slows down print speed for small print surfaces. This, of course, screwed with every test/calibration object.

The path to the offending setting is:
Slic3r
Filament Settings
Cooling
Cooling Thresholds
Slow down ....
and
Min print speed