I will relate a recent experience.
A lady friend brought her Giant Upland Ladies (Purple) MTB over for some upgrading. I changed the tyres to 26x1.5 city semi slicks, cut the riser bars down to 560mm wide, fitted a suspension seat post and Sell Italia Lady Saddle, and ordered the new 11-28 cassette from Shimano.
She swims at the same pool/squad as myself so I decided to ride it to the pool and show her. The test ride with the 14-28 cassette, well, too slow, so I took the MTB wheels off and fitted a pair of my 650c training wheels and an 8 speed 11-32 HG30 cassette, set the derailleur up to shift the high 7 (11-28), pumped up the race tyres and rode off to the pool.
Well, this was the fun ride of the year!! I could keep up with everything!! But, did it embarrass the road bike riders.
A purple ladies MTB keeping up on the bike path! Well, I beat my normal training time for the journey of 50-55 minutes by 2-7 minutes.
So, it does not matter what you ride, just how you ride it, Enjoy!!