Why is my Mcintosh MC-2155 slow?

McIntosh amplifiers normally sound slower than other solid state amplifiers so there is nothing wrong with your 2155. However, there is way that you can change the way it sounded. The MC 2155 has an input sensitivity switch of 0.75V/2.5V. Set it at 0.75V will reduce the negative feedback and increase the gain of the MC 2155. With higher gain, it will sound more dynamic and forward. You can then use the volume control on the front panel to fine tune the best balance between the smoothness and the transient speed.

You can also consider Thiel speakers. They are neutral, transparent enough that you likely will keep hearing faults in the chain before it during the first month, and have excellent soundstage if given the space. Given your room size limitation, I would consider those in the 2.* series.

Since Thiel speakers are rather neutral, a McIntosh amp will bring it to the warm side and give them a nice body in the mid-range. MC 2155 should have enough power to drive those under 3.* series. Thiel speakers do not allow bi-wiring, which goes nicely with those narrow McIntosh speaker connection strips. Last but not the least, Thiel has excellent customer supports. Being able to access manufacturer directly instead of importer makes a big difference when you need service.