Why 2 Table Scans of Same Table Have Huge Differences in Elapsed Time SQL Server
Logical reads is only an indicator of how much data SQL Server reads from disk (or the buffer pool). There are a bunch of other factors that can contribute to a long-running query.
The most obvious suspect is how much time it takes to move the data from your server to Management Studio and/or the time it takes to render the data in a table. To eliminate those two, try inserting the results in a new temp table:
SELECT a, b INTO #new_table FROM #temp_table;

You can also set
SET STATISTICS TIME ON;

to see the amount of CPU and Elapsed times respectively.