Those 128 physical reads were not necessarily done sequentially. They may have been spread all over the place. Without the query plan we cannot tell.
The first thing to do here is to get the query plan and see if that looks reasonable.
In order to measure disk performance, use OS commands like 'iostat', or use the MDA tables (see www.sypron.nl/mda).
MDA tables (like monOpenObjectActivity) are also the right way to figure out for which tables those I/Os were done. sp_sysmon will not tell you this.
Everything may look perfect though, and there may be other reasons why it may take longer than expected. For example, your query may have been blocked for a short while by another user which makes its total response time slower than expected (MDA tables will again help here).
Bottom line: you may need to look at many aspects in order to come to a good diagnosis of why a response time is slower than you'd hoped to see.