I wouldn't be surprised with a gradual drop before a service, but it should be back to normal, or better, afterwards.
Go back to the guys who did it, but have a check first that they have used a genuine fuel filter, and the correct plugs etc. A pattern filter could be more restrictive, and most Suzi's are particular about their NGK plugs.
Ask if they removed any linkages to do the service that may not have gone back exactly the same.
Ask them to explain why.
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