During my current run Shroom did have at least four different Speed values with no chips, upgrades, steles, or anything else visibly impacting it. She started on 7.7, went down to 6.5 (which seems to be the correct value, most in line with the other heroes), back up to, I think, 7.3 at the beginning of the second floor and down to 7 after meeting the first monsters of the zone. All values while her being the active hero (so not related to my last comment in this reported bug).
Step by step of the bug
Step 1
Play with Shroom
Step 2
Progress through the game and check her Speed stat
During my current run Shroom did have at least four different Speed values with no chips, upgrades, steles, or anything else visibly impacting it. She started on 7.7, went down to 6.5 (which seems to be the correct value, most in line with the other heroes), back up to, I think, 7.3 at the beginning of the second floor and down to 7 after meeting the first monsters of the zone. All values while her being the active hero (so not related to my last comment in this reported bug).