You can temporarily set the product price to 0.00 and then Add to Cart that product and Checkout to reach the thank-you page directly.
If you would rather not leave your product price active at 0.00 even for a moment for testing purposes, you can temporarily enable the File Download setting for the product(s) you wish to test, even if there is no actual file to download. This setting includes a download link in the thank-you email, which link actually brings up the product's thank-you page (where any actual download link would normally appear). Then you can use your Seller Admin > "Send free thank-you email/download link" to send yourself a thank-you email with that link to view your thank-you page. Once you have finished testing your thank-you page, you can disable the File Download setting for that product.
Does the thank-you page delay only happen with Google Checkout? That sometimes happens when a payment is still being processed when the buyer clicks the return/continue link on Google's checkout page. Note that PayPal Pro (not regular PayPal) is the only processor that auto-refreshes to a thank-you page. All other payment processors only offer a return/continue link on their checkout page, so the customer would only see the thank-you page if they manually click that link, and only if their payment had already completed pretty much instantly. If their payment was not yet complete, then the thank-you page link would only bring up a generic message asking them to check their email for further info, since we would not want to issue any download links or purchased codes (typically presented on the thank-you page) until their payment clears as good and complete.