Arriving Late to Shaharith

If I am davening Shacharit with a minyan and I am in the middle of saying Ashrei before Uva Letziyon, but the minyan is davening very fast and has already begun saying Uva Letzion, should I stop saying Ashrei and begin saying Uva Letzion?

Under the circumstances which you describe you should continue to say Ashrei at a pace which allows you to have
kavannah. One should pause from saying Ashrei for the three points at which the tzibbur must pronounce in unison with
the shaliach tzibbur-
after veatah kadosh-”kadosh, kadosh, kadosh”
after vatisaeni-”baruch kevod adonai mimkomo”
after berich yekara-”adonai yimloch leolam vaed.”
Thus, one continues to say Ashrei and joins in unison with the tzibbur at the appropriate points which I have described.