Is there a speed difference in ordering by int vs. float?

It depends. You didn't specify the RDBMS so I can only speak to SQL Server specifically but data types have different storage costs associated with them. Ints range from 1 to 8 bytes, Decimals are 5-17 and floats are 4 to 8 bytes.

The RDBMS will need to read data pages off disk to find your data (worst case) and they can only fit so many rows on an 8k page of data. So, if you have 17 byte decimals, you're going to get 1/17th the amount of rows read off disk per read than you could have if you sized your data correctly and used a tinyint with a 1 byte cost to store X.

That storage cost will have a cascading effect when you go to sort (order by) your data. It will attempt to sort in memory but if you have a bazillion rows and are starved for memory it may dump to temp storage for the sort and you're paying that cost over and over.

Indexes may help as the data can be stored in a sorted manner but again, if getting that data into memory may not be as efficient for obese data types.

[edit]

@Bohemian makes a fine point about the CPU efficiency of integer vs floating point comparisons but it is amazingly rare for the CPU to be spiked on a database server. You are far more likely to be constrained by the disk IO subsystem and memory which is why my answer focuses on the speed difference between getting that data into the engine for it to perform the sort operation vs the CPU cost of comparison.