Looking on the internet deeply has found these results:

speak more slowly is the most popular phrase on the web.

speak slowlier 11,120 results on the web	speak more slowly 1205,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Speak to nobody along the road. I also speak a foreign language, Charlie. Guess who offered to speak instead. Speak now or forever sing soprano. And as movement in copper speak means 'slower than a wet week'... So speak to the Council; speak with those who are blocking it. That question is being addressed as we speak. How you speak without saying anything.	Yukkuri hanashite kudasai. Mr President, I shall try to speak more slowly this time because last time I gave the interpreters a hard time by speaking at a Mediterranean tempo. It is a long answer, and I am a little afraid that Members will not be patient to listen until the end of my speech, but I will speak more slowly. (The President asked the speaker to speak more slowly so as to permit interpretation) Please speak more slowly Speak more slowly so that we can understand you. All right, I'll speak more slowly. Do you understand what I'm saying, or shall I speak more slowly?

speak slowlier

11,120 results on the web

speak more slowly

1205,000 results on the web

More popular!

Yukkuri hanashite kudasai.
Mr President, I shall try to speak more slowly this time because last time I gave the interpreters a hard time by speaking at a Mediterranean tempo.
It is a long answer, and I am a little afraid that Members will not be patient to listen until the end of my speech, but I will speak more slowly.
(The President asked the speaker to speak more slowly so as to permit interpretation)
Please speak more slowly
Speak more slowly so that we can understand you.
All right, I'll speak more slowly.
Do you understand what I'm saying, or shall I speak more slowly?