Reading Arrowsmith is kind of like talking to a favorite grandpa. The expressions can be weirdly old-fashioned. There are a lot of repetitions of "Say!" and "You bet!" but overall nothing worse than you'd find in The Great Gatsby.
What will really get you (if anything) are the long-winded sentences. We told you reading this novel was a little like talking to your grandpa. Sinclair Lewis can meander, but that's less a particular quirk of his and more of a sign of the times in which he was writing. Remember that this came out a full year before Hemingway's first novel, and Hemingway's short sentences were considered insanely revolutionary.
So just go with the (sometimes meandering) flow. Most of the time Sinclair Lewis is very straightforward, only occasionally getting poetic with gems like:
They sped out of the capital by white shell roads agonizing to the sun-poisoned eyes; they left the dusty shanties of suburban Yamtown for a land cool with bamboo groves and palmettoes, thick with sugar-cane. (34.1.3)
And that's about as challenging as it gets, flowery language-wise. Lewis didn't become a phenomenally popular writer for nothing: people could understand what he was writing.